The FBI and other police agencies don't need a search warrant to track the locations of Americans' cell phones, a federal appeals court ruled on Tuesday in a precedent-setting decision.

In the first decision of its kind, a Philadelphia appeals court agreed with the Obama administration that no search warrant--signed by a judge based on a belief that there was probable cause to suspect criminal activity--was necessary for police to obtain logs showing where a cell phone user had traveled.

A three-judge panel of the Third Circuit said (PDF) tracking cell phones "does not require the traditional probable-cause determination" enshrined in the Fourth Amendment, which prohibits government agencies from conducting "unreasonable" searches. The court's decision, however, was focused on which federal privacy statutes apply.

But the panel sided with civil-liberties groups on an important point: it agreed that, in at least some cases, judges may require investigators to obtain a search warrant. That is, however, "an option to be used sparingly," the court said.

Some questions are likely to be resolved in future proceedings, once the case returns to a lower court. "It is still an open question as to whether the Fourth Amendment applies to cell phone records," Electronic Frontier Foundation attorney Kevin Bankston said after the ruling. "This decision does not definitively answer the question of the Fourth Amendment status of cell phone [location records]."

In this case, U.S. Magistrate Judge Lisa Lenihan denied the Justice Department's attempt to obtain stored location data without a search warrant, saying federal privacy law prohibited it. Lenihan's ruling, in effect, would require police to obtain a search warrant based on probable cause--a more privacy-protective standard.

The Obama administration had argued that warrantless tracking is permissible because Americans enjoy no "reasonable expectation of privacy" in their--or at least their cell phones'--whereabouts. U.S. Department of Justice lawyers said "a customer's Fourth Amendment rights are not violated when the phone company reveals to the government its own records" that show where a mobile device placed and received calls.

Lenihan had required the Justice Department to demonstrate "probable cause," a standard used in search warrants. But the three-judge panel rejected that idea, saying Lenihan "erred" and the relevant requirement is a "lesser one than probable cause" that is less privacy-protective.

The Bureau of Alcohol, Tobacco, Firearms, and Explosives, or ATF, had told Lenihan that it needed historical (meaning stored, not future) phone location information because a set of suspects "use their wireless telephones to arrange meetings and transactions in furtherance of their drug-trafficking activities." The name of the mobile-service provider is not public.

The ACLU, the Electronic Frontier Foundation, and the Center for Democracy and Technology had argued (PDF) that because cell phone information "is protected by the Fourth Amendment," a search warrant was necessary. The court did not squarely address that question in Tuesday's ruling.

EFF's Bankston said it was encouraging to see a ruling that allowed judges to demand search warrants at least in some cases. "The court explicitly refused to set a boundary for the court's discretion," he said. "It clarifies that judges have the discretion that the government has long argued they don't have."

The Justice Department did not immediately respond to questions from CNET about whether it would appeal that portion of the ruling to the Supreme Court or seek a review from the Third Circuit.

Not long ago, the concept of tracking cell phones would have been the stuff of spy movies. In 1998's "Enemy of the State," Gene Hackman warned that the National Security Agency has "been in bed with the entire telecommunications industry since the '40s--they've infected everything." After a decade of appearances in "24" and "Live Free or Die Hard," location tracking has become such a trope that it was satirized in a scene with Seth Rogen from "Pineapple Express" (2008).

Cell phone tracking comes in two forms: police obtaining retrospective historical data kept by mobile providers for their own billing purposes that is typically not very detailed, or prospective tracking--which CNET was the first to report in a 2005 article--that reveals the minute-by-minute location of a handset or mobile device.

The Obama administration argues that no search warrant is necessary; it says what's needed is only a 2703(d) order, which requires law enforcement to show that the records are "relevant and material to an ongoing criminal investigation."

About the author

Declan McCullagh is the chief political correspondent for CNET. You can e-mail him or follow him on Twitter as declanm. Declan previously was a reporter for Time and the Washington bureau chief for Wired and wrote the Taking Liberties section and Other People's Money column for CBS News' Web site.
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